Attrition resistant proppant composite and its composition matters

ABSTRACT

A hydraulic fracturing and gravel packing proppant composite with protectant on the surface of the proppant and the composition matters of the protectant and proppant. The surface protectant reduces the generation of dust/fines from the proppant caused by abrasion and impingement during transportation and conveyance, particularly pneumatic transfer.

CROSS REFERENCE

This application is based on and claims priority to U.S. patentapplication Ser. No. 14/669,815 filed Mar. 26, 2015 and U.S. PatentApplication No. 61/971,995 filed Mar. 28, 2014.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates generally to a proppant composite, and moreparticularly, but not by way of limitation, to an attrition resistantproppant composite for use in hydraulic fracturing.

Description of the Related Art.

Hydraulic fracturing is commonly used in oil and gas production toaccess carbon trapped in impermeable geological formations. The processinvolves injecting a highly pressurized fluid, typically containingwater or another carrier, chemicals, and proppants, into a wellbore,which causes the underlying rock to crack. The proppants in the fluidthen stay in the cracks in the rock and hold the cracks open, allowingunderlying hydrocarbons to flow through the cracks into the wellbore forcollection.

Proppants like quartz sand, resin coated sand, ceramics, and materialslike bauxite used to make ceramics, for example, are now commonly usedin hydraulic fracturing to increase the production of oil and gas fromsubterranean formations. However, all of these proppants tend togenerate dust/fines upon shipping and handling before they are pumpedinto the well for fracturing. When quartz sand is employed forfracturing, for example, attrition and impingement among quartzparticulates and between quartz particulates and the walls of thecontainer occurs during shipping to the fracturing job site. Thisattrition and impingement are greatly increased during transfer andunloading of the material. Dust/fines are created during bin loading,belt transfer, blender loading, release from multi-sander operations,release from stingers, dust ejection from open fill ports, and otherhandling operations with potential adverse health and environmentaleffects. In particular, pneumatic air unloading creates high levels ofdust/fines.

Dust/fines are microscopic particulate matter that can be suspended inthe air. Such particulate matter occurs naturally, and can also beman-made. This invention is directed to limiting the creation ofdust/fines through the use of attrition resistant proppant in transportand hydraulic fracturing and other applications. Respirable particlesare a particular concern for the health and safety of workers and otherpersons who come into contact with dust/fines. These airborneparticulates are potentially hazardous because of their ability topenetrate deep into the alveoli of the lungs. In particular, chronic orexcessive exposure to respirable crystalline silica such as quartz hasbeen shown to cause pneumoconiosis, commonly known as silicosis.

Within the class of dust/fines, respirable particles are those smallenough to enter the alveoli of the lungs and generally includeparticulates with a diameter of 10 micrometers (or microns) or less. Asparticle size drops below 10 microns, the probability of particlesbecoming trapped in the alveoli increases. Although the presentinvention is successful in the suppression of dust/fines with a diameterof 10 microns or less (including fines with a diameter of 2.5 microns orless), the invention may also suppress other suspended particulatematter that may be larger or sub-micron in size. As greater medical andenvironmental awareness of the consequence of respirable suspendedparticles is known, this invention will continue to apply to thesuppression of dust/fines in hydraulic fracturing and other applicationsas those terms may be understood in future practice or regulation.

With regard to the current regulatory environment, the U.S. OccupationalSafety and Health Administration (OSHA) is an agency of the U.S.Department of Labor empowered to assure safe and healthful workingconditions by setting and enforcing workplace standards. OSHAestablishes Permissible Exposure Limits (PELs) for many chemicalsubstances in 29 CFR 1910.1000. OSHA's current PEL for respirable silicadust in General Industry is found in 29 CFR 1910.1000 TABLE Z-3 and theOSHA Technical Manual (OTM) Section II: Chapter 1 Appendix J, SampleCalculations for Crystalline Silica, including the followingformulation, Equation 6 from Section III.K.2 of Appendix J:

PEL (mg/m ³)=(10 mg/m ³)/(2+% respirable quartz)

Therefore, for a dust containing 100% quartz, the PEL is 10/(100+2), orroughly 0.1 mg/m³. The term “respirable quartz” includes dustscontaining greater than one percent quartz with a particle size smallenough to reach the alveolar space in the lungs, or less than 10 μm inaerodynamic diameter. Dust exposures are expressed as either a particleconcentration (for example, millions of particles per cubic foot of airor mppcf) or a gravimetric concentration (unit mass of particles pervolume of air, such as mg/m³). OSHA's regulatory authority is subject toadministrative rulemaking process which includes public comment andreview. This administrative and political process can result in new orrevised standards that take years to be developed, finalized, andpromulgated as a standard. The employer's efforts to control silicaexposures below the PEL, as addressed by the present invention, willbecome more difficult if OSHA's proposed rulemaking lowers the PEL to0.05 milligrams of respirable crystalline silica per cubic meter of air(0.05 mg/m³), as indicated in OSHA's Proposed Rule to the FederalRegister on Sep. 12, 2013.

OSHA recognizes that many of its PELs are outdated and that revising thecurrent PELs is a lengthy and complicated process. As such, OSHArecommends that employers consider using alternative occupationalexposure limits (i.e., NIOSH Recommended Exposure Limits (RELs) and theACGIH TLVs). Regarding best industry industrial hygiene practices, theAmerican Conference of Governmental Industrial Hygienists (ACGIH) is amember-based organization dedicated to the industrial hygiene andoccupational health and safety industries. The ACGIH annually publishesthe ACGIH Guide to Occupational Exposure Values, considered the standardresource for occupational exposure limits in the United States. TheACGIH Threshold Limit Value (TLV) for an eight-hour time weightedaverage (TWA) workshift exposure to respirable crystalline silica, asincluded in the 2015 Guide to Occupational Exposure Values and cited inOSHA's 29 CFR 1910.1200 Annotated TABLE Z-3 Mineral Dusts, is 0.025mg/m3 for a-quartz. The National Institute for Occupational Safety andHealth (NIOSH) is part of the Center for Disease Control and Prevention(CDC) within the U.S. Department of Health and Human Services. Amongother things, NIOSH is responsible for conducting research and makingrecommendations for the prevention of work-related injury and illnessbased on the best available scientific data. The currently publishedNIOSH Recommended Exposure Level (REL) for a TWA associated with up to aten-hour workday during a 40-hour workweek is 0.05 mg/m³ for crystallinesilica as respirable dust. In addition to these U.S. agencies, foreignagencies are also involved in setting workplace standards andrecommendations, including the Scientific Committee on OccupationalExposure Limits (SCOEL) and Institut fur Arbeitsschutz der DeutschenGesetzlichen Unfallversicherung (IFA), which advise the EuropeanCommission regarding occupational exposure limits for chemicals in theworkplace, and the Workplace Exposure Standards for AirborneContaminants published in association with the Australian Work Healthand Safety Act. The present invention is directed to complying with allthese various standards and recommendations, as well as self-imposedstandards that may exceed these requirements.

Proppant fines can also cause problems in the recovery of oil and gas.Fines are smaller than whole proppant and thus less effective atpropping the cracks open for the oil and gas to flow through.Furthermore, they tend to clog the cracks, inhibiting the flow ofhydrocarbons and reducing the productivity of the well.

Previous coated proppants have been aimed primarily at increasing thecrush strength of the proppant, with dust control considered only as asecondary benefit. Increasing crush strength can be achieved by coatingthe proppant with resin. This is very expensive, however, and thus anundesirable solution to increase attrition resistance. Other coatingsare aimed at controlling dust by agglomerating small dust particles,rather than preventing dust from forming in the first place.

Based on the foregoing, it is desirable to provide a proppant with lowdust/fines subsequent to shipping and handling and, especially, duringpneumatic air unloading.

It is further desirable for such a proppant to be a new kind of proppantfor the hydraulic fracturing industry.

It is further desirable for such a proppant to allow users of theproppant to be in compliance with OSHA PEL, NIOSH REL, and similarrequirements subsequent to shipping and handling and upon pneumatic airunloading, which will better protect workers and prevent nuisancedusting which might disturb the local community near the sand plant, thetransload facilities, or the fracturing job site.

It is further desirable for such a proppant to facilitate compliancewith a reduced OSHA PEL, NIOSH REL, or similar regulations if requiredin the future.

It is further desirable for such a proppant to be less expensive toproduce than resin coated proppant.

It is further desirable for such a proppant to prevent dust formationrather than solely agglomerating existing dust.

SUMMARY OF THE INVENTION

In general, in a first aspect, the invention relates to a surfacemodified proppant comprising a proppant; and a chemical coating at leastpartially covering the proppant. The chemical coating may benon-petroleum-based, glycerin-based, propylene glycol-based, or acombination thereof. Additionally or alternately, the surface modifiedproppant may have a Turbidity Reduction Factor greater than about 40%and a Respirable Dust Reduction Factor greater than about 70%.Additionally or alternately, the chemical coating may not be a thermosetpolymer, not be an ionic polymer, not be a thermoplastic elastomer, andnot be a hydrogel. The coating may increase the attrition-resistance ofthe proppant. The coating may additionally or alternately reduce thegeneration of dust/fines of the proppant upon shipping, handling,pneumatic air unloading, or combinations thereof.

The proppant prior to coating application may be substantially dustfree, and may be a raw substrate, including sand, ceramic, or compositematerial, minerals, ground shells, resin coated proppants, orcombinations thereof. The coating may be non-toxic. The coating may notbe an ionic polymer. The coating may be less than 2 wt. % of the surfacemodified proppant, less than 1 wt. % of the surface modified proppant,or 0.05 to 0.20 wt. % of the surface modified proppant. The coating maybe glycerin-based coating, vegetable oil/wax-based coating, tall oilpitch based coating, alkyl ester based coating, or a combinationthereof. If the coating is alkyl ester based coating, the coating may belower alkyl ester based, particularly methyl and ethyl ester based.

The chemical coating may be applied to the proppant through spray,mechanical mixing, non-mechanical mixing, or a combination thereof. Thecoating may comprise multiple coatings, and the coatings may be appliedsequentially or simultaneously onto the proppant. The multiple coatingsmay comprise a first coating and a second coating and the first coatingmay have a different chemical composition than the second coating.Alternately, the chemical composition of the first coating may be thesame as the chemical composition of the second coating. The coating maybe applied to the proppant prior to the proppant being used. The coatingmay comprise a thick layer of coating, a thin layer of coating, or apartial layer of coating.

The surface modified proppant may further comprise a chemical marker,such as a colorant, a UV dye, a conductivity enhancing chemical, or acombination thereof. Additionally or alternately, the surface modifiedproppant may further comprise a frac fluid delay-crosslinking agent,which may be betaine, gluconate, polyglycol, or a combination thereof.The coating may not require curing or drying.

In a second aspect, the invention relates to a method of producing asurface modified proppant, the method comprising adding a chemicalcoating to a proppant, where the chemical coating is not a thermosetpolymer, not an ionic polymer, not a thermoplastic elastomer, and not ahydrogel, or more specifically non-petroleum-based, glycerin-based,propylene glycol-based, or a combination thereof, and mixing the coatingand the proppant or spraying the coating on the proppant without mixingto produce the surface modified proppant. The coating may not be anionic polymer. The coating may be less than 2 wt. % of the surfacemodified proppant, less than 1 wt. % of the surface modified proppant,or 0.05 to 0.20 wt. % of the surface modified proppant. The mixing mayoccur in a powered continuous mechanical blender, a powered batchmechanical blender, a static mixer, or a combination thereof. The methodmay further comprise adding a second chemical coating to the surfacemodified proppant and mixing the second chemical coating and the surfacemodified proppant.

In a third aspect, the invention relates to a method of reducingproppant attrition anywhere in a proppant supply chain, the methodcomprising using a surface modified proppant. The surface modifiedproppant may comprise a proppant and a chemical coating at leastpartially covering the proppant, where the chemical coating is not athermoset polymer, not an ionic polymer, not a thermoplastic elastomer,and not a hydrogel, or more specifically non-petroleum-based,glycerin-based, propylene glycol-based, or a combination thereof.

The proppant may be a raw substrate, including sand, ceramic, orcomposite material, composites, minerals, ground shells, resin coatedproppants, or combinations thereof. The chemical coating may beglycerin-based coating, vegetable oil/wax-based coating, tall oil pitchbased coating, alkyl ester based coating, or a combination thereof. Thecoating may not be an ionic polymer. The coating may be less than 2 wt.% of the surface modified proppant, less than 1 wt. % of the surfacemodified proppant, or 0.05 to 0.20 wt. % of the surface modifiedproppant.

The chemical coating may be applied to the proppant through mechanicalmixing, spray, non-mechanical mixing, or a combination thereof. Thecoating may comprise multiple coatings applied simultaneously orsequentially onto the proppant. The multiple coatings may comprise afirst coating and a second coating and the first coating may have adifferent chemical composition than the second coating or the chemicalcomposition of the first coating may be the same as the chemicalcomposition of the second coating. The coating may be applied to theproppant prior to the proppant being used.

The surface modified proppant may further comprise a chemical marker,such as a colorant, a UV dye, a conductivity enhancing chemical, or acombination thereof. The surface modified proppant may further comprisea frac fluid delay-crosslinking agent, which may be betaine, gluconate,polyglycol, or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of a surface modified proppant for use in afracturing process, where the surface modified proppant has a thickcoating;

FIG. 2 shows a cross section of a surface modified proppant for use in afracturing process, where the surface modified proppant has a thincoating;

FIG. 3 shows a cross section of a surface modified proppant for use in afracturing process, where the surface modified proppant has a partialcoating;

FIG. 4 is a flow chart for a process for making a surface modifiedproppant at scale;

FIG. 5 is a diagram of the use of a modified proppant in hydraulicfracturing;

FIG. 5a is a close-up of a portion of the diagram of FIG. 5;

FIG. 6 is a chart showing the reduction in dust/fines generation duringabrasion/attrition, as measured by turbidity, of the surface modifiedproppants described in Examples 1 through 12 and 16 through 21;

FIG. 7 is a chart showing the reduction in dust/fines generation duringabrasion/attrition, as measured by turbidity, in a 12.5 hour ball millabrasion test;

FIG. 8 is a chart showing the respirable quartz dust levels of uncoatedfrac sand and coated frac sand upon pneumatic air unloading;

FIG. 9 is a chart showing the dust/fines reduction, as measured byturbidity, of the surface modified proppants described in Examples 22through 26;

FIG. 10 is a chart showing the particle size distribution of the systemsdescribed in Example 28;

FIG. 11 is a chart showing the dust/fines reduction, as measured byturbidity, of the surface modified proppants described in Examples 28through 30;

FIG. 12 is a chart showing the total respirable dust reduction, asmeasured in percentage, of the surface modified proppants described inExamples 1 and 4 as tested in Example 15, Example 33, and Example 34;and

FIG. 13 is a chart showing the total percent turbidity reduction factoracross many of the samples described in Examples 1 through 14, 16through 25, and 28 through 30.

Other advantages and features will be apparent from the followingdescription and from the claims.

DETAILED DESCRIPTION OF THE INVENTION

The devices and methods discussed herein are merely illustrative ofspecific manners in which to make and use this invention and are not tobe interpreted as limiting in scope.

While the devices and methods have been described with a certain degreeof particularity, it is to be noted that many modifications may be madein the details of the construction and the arrangement of the devicesand components without departing from the spirit and scope of thisdisclosure. It is understood that the devices and methods are notlimited to the embodiments set forth herein for purposes ofexemplification.

In general, in a first aspect, the invention relates to an attritionresistant proppant composite and its composition matters. In testing, athick layer, a thin layer, or partial covering of glycerin-basedcoating, vegetable oil-based coating, or tall oil pitch based coatingsurprisingly was able to increase the attrition/impingement resistanceof the surface modified proppant and was able to greatly reduce therespirable dust/fines level upon pneumatic air unloading of such asurface modified proppant composite. Such a discovery is significant inprotecting the workers who are exposed to proppant dust. The green andsustainable nature of such chemical coatings also is able to betterprotect our environment and water resources.

In addition to worker safety issues related to OSHA compliance and NIOSHguidelines for airborne particulate matter, the proppant composite maybe used downhole in the hydraulic fracturing context. The material mayalso be used as further described below as an industrial, construction,or playground sand or in similar contexts. The green and sustainablenature of the chemical coating is important. It can avoid thecontamination of water either in above ground collection areas (such asponds, streams, or runoff from a site) as well as groundwater. Inaddition to OSHA and NIOSH, it is possible that the U.S. EnvironmentalProtection Agency (EPA) or other agencies will introduce regulationsthat encourage, or require, the use of biologically and environmentallyfriendly materials like the surface modified proppants described herein.

The proppant may be a surface modified proppant for use in a fracturingprocess. A cross section of the proppant may be seen in FIGS. 1, 2, and3. The modified proppant 100 may offer low dust/fines upon shipping andhandling and upon pneumatic air unloading at the fracturing job site.Environmentally friendly chemicals/coatings 120 such as glycerin-basedcoating formula, vegetable oil-based formula, or tall oil pitch basedcoating formula may be employed to modify the proppant. The proppant 110may be sand, such as quartz sand, resin-coated quartz sand, beach sand,golf sand, coral sand, volcanic ash, glass sand, gypsum sand, Ooid sand,silica sand, black sand, green sand, desert sand, lithic sand, biogenicsand, garnet sand, olivine sand, heavy mineral sand, continental sand,quartz sand, or other types of sand; or ceramics, materials used to makeceramics such as bauxite, light weight ceramics, or resin-coatedceramics, typically used in the fracturing industry, or other suitableparticulate materials such as ground quartz, ground shells, etc. Thesurface modified low-dust generating composite system can also beapplied to other dust generating particulates like talc, feldspar,diatomite, kaolin, ground quartz, beach sand, playground sand, fumesilica, golf course sand, etc. The proppant 110 is shown as round orspherical in FIGS. 1, 2, and 3, but may be of any geometric shapewithout departing from the present invention.

The chemical coating 120 may not be a thermoset polymer, an ionicpolymer, a thermoplastic elastomer, or a hydrogel. The chemical coating120 may be non-petroleum based, glycerin-based, propylene glycol-based,or a combination thereof. More particularly, the chemical coating may beglycerin-based, vegetable oil based, tall oil pitch based, methyl and/orethyl ester based, or a combination thereof, or may be mineral oil orother suitable coating. The coating may not be an ionic polymer, wherean ionic polymer includes polyanionic and polycationic polymers,including synthetic polymer, biopolymer, or modified biopolymercomprising carboxy, sulfo, sulfato, phosphono, or phosphate groups or amixture thereof or a salt thereof, or primary, secondary, or tertiaryamines or quaternary ammonium groups or suitable salt thereof in thebackbone or as substituents. The chemical coating may be environmentallyfriendly and may be non-toxic to humans and/or animals. The coating maynot require curing or drying. The coating 120 may not be an additive orsecondary coating used in conjunction with a different coating for adifferent purpose, but may be used alone as a primary coating element.

Petroleum-based coatings or treatments may alter the environmentalfriendliness and toxicity profile of the modified proppant system,including as to airborne dust/fines in handling the material prior toits downhole introduction as well as residual environmentalcontamination following downhole application. By way of non-exhaustiveexclusion, the modified proppant 100 may not include a petroleum-basedcoating other than glycerin or propylene glycol, a surface hydrogellayer, synthetic polymer layer, silane functional agent layer, syntheticresin layer, thermoplastic elastomer, or other coating based on apetroleum fraction or a polymer made from a petroleum fraction monomer.Other coatings excluded from the present invention include tackifyingagents including polyamides and polyacids, organic coatings of thevariety of thermoplastic elastomers or thermosetting polymers,polyurethane, cured isocyanate functional components, glycerol rosinester or pentaerythritol rosin ester, phenol-aldehyde novolac polymer,polycarbodiimide, epoxy, or viscoelastic surfactants. Such additional oralternative coatings are sometimes used to suspend a proppant in aslurry, deliver a proppant into a fracture, encourage conductivity(i.e., in this context the flow of hydrocarbons, not electricity),withstand structural pressure (i.e., crush strength), or for otherdownhole purposes. It is possible that the modified proppant 100 of thepresent invention may be used in combination with these other materialsystems to address multiple competing concerns in a hydraulic fracturingor related context or for other downhole purposes (i.e., to reduce theflowback of proppant).

The coating may be applied prior to the use of the proppant. Thus, thecoating may be applied to new, substantially dust-free proppant. Assuch, the coating may primarily prevent dust formation by preventingproppant attrition rather than merely suppressing existing dust. Given aproppant with a particular particle size, uncoated proppant may breakapart during shipping, handling, and other use. The coated proppant maymaintain the same particle size, with the coating preventing theproppant particles from breaking. This coated proppant may bedistinguished from a coating applied to dusty aggregate to agglomeratethe existing dust and prevent the existing dust from becoming airborne.Proppant may be considered substantially dust free if it has a turbidityof less than 200, preferably less than 150, more preferably less than100, and most preferably less than 50. Turbidity is the cloudiness orhaziness of a fluid caused by suspended solids that may be invisible tothe naked eye. Dust/fines suspended in water are similar in particlesize to respirable dust particles that may present breathing hazards.Substantially dust free proppant may be proppant produced at a sand minewhere the sand was washed, dried, screened, and optionally stored in asilo. The turbidity of the proppant may depend on the grade. Forexample, samples of 40/70 and #100 grades may have a higher turbiditythan samples of 20/40 and 16/30 grades, even when all of the samples aresubstantially dust free.

In a laboratory setting, the coating may be applied by dropwise additionof the coating to 200 g of a chosen particulate material at roomtemperature. The coating and proppant may be well mixed by hand with astainless spatula for five minutes until the coating is well distributedto the proppant. Alternately, the mixing can be accomplished by otherwell-known mechanical mixing methods.

FIG. 4 is a flow chart for a process for making a surface modifiedproppant at scale. The process may begin with the precursor material,the proppant 110, in Step 210. A first layer of the chemical coating 120may then be applied in Step 220. For industrial use, the coating may beapplied using spray, a powered continuous mechanical blender, a poweredbatch mechanical blender, a static mixer, or a combination thereof, orusing other mixing or application methods as desired. After application,the chemical coating 120 may be less than 2 wt. % of the surfacemodified proppant 100, less than 1 wt. % of the surface modifiedproppant 100, or most preferably 0.05 to 0.20 wt. % of the surfacemodified proppant 100.

When a powered continuous mechanical blender is used in Step 220, thepowered continuous mechanical blender may have rotating shaft-mountedpaddles, pins, a ribbon or ribbons, or any combination thereof and maybe powered with a motor, engine, or other drive system. Additionally oralternately, a rotating drum or other vessel, which may comprise mixingflights, buckets, plates, dams, etc., may be utilized. The coating maybe applied to the proppant upstream of the powered continuous mechanicalblender, or during entry of the proppant into the powered continuousmechanical blender, or immediately after the proppant enters the poweredcontinuous mechanical blender. The coating application point may beconfigured in such a way to establish a falling curtain pattern ofproppant flow where the coating is applied to allow for more efficientdistributive application of the coating. Alternately or additionally, aspray nozzle system may be utilized for more efficient distributiveapplication of the coating. The rotating paddles, pins, and/or ribbonsmay facilitate mixing of the proppant and coating and may convey thecoated proppant to the discharge end of the powered continuousmechanical blender. The powered continuous mechanical blender may have asingle rotating shaft or may have two or more rotating shafts. This mayresult in a continuous-process blending procedure to facilitate evenspreading of the coating product onto the proppant. Alternately, thebelts, drops, and conveying at a sand plant or a transloader, or at anysite that conveys the sand, may provide adequate if not optimal mixingof the coating.

A powered batch mechanical blender may use a motor, engine, or otherdrive system to facilitate mixing the proppant and coating. The coatingmay be applied to the proppant upstream of the powered batch mechanicalblender, or during entry of the proppant into the powered batchmechanical blender, or immediately after the proppant enters the poweredbatch mechanical blender. The coating application point may be locatedor configured in such a way to establish a falling curtain pattern ofproppant flow where the coating is applied to allow for more efficientdistributive application of the coating. Alternately or additionally, aspray nozzle system may be utilized for more efficient distributiveapplication of the coating. The rotating paddles, pins, and/or ribbonsmay facilitate mixing of the proppant and coating. The powered batchmechanical blender may have a single rotating shaft or may have two ormore rotating shafts. After blending, the coated proppant may exit thepowered batch mechanical blender and the process may be repeated. Thismay result in a batch-process blending procedure to facilitate evenspreading of the coating product onto the proppant.

A static mixer may use non-powered means to mix the proppant andcoating. Proppant may be gravity-fed through the static mixer. Thecoating may be applied to the proppant upstream of the static mixer, orduring entry of the proppant into the static mixer, or immediately afterthe proppant enters the static mixer. The coating application point maybe located or configured in such a way to establish a falling curtainpattern of proppant flow upstream of the static mixer, at the feed endof the static mixer, or immediately after proppant enters the staticmixer to allow for more efficient distributive application of thecoating. Alternately or additionally, a spray nozzle system may beutilized for more efficient distributive application of the coating.Baffles, diverters, plates, ladder rungs, etc. may be installed insidethe static mixer to facilitate mixing of the proppant and coating. Thismay result in a continuous-process blending procedure to facilitate evenspreading of the coating product onto the proppant.

In Step 230, a decision point may be reached where additional coatinglayers may be applied, if desired. Each coating may be appliedsequentially onto the aggregate. Each of the multiple coatings may be alayer of the same type of coating, or each layer may be a different typeof coating, or a combination thereof. Each layer of coating may beapplied using any one or more of the application processes describedabove in Step 220.

In Step 240, a decision point may be reached where one or moreadditional chemical modifications may be performed on the modifiedproppant. Chemical markers like colorants, UV dyes, and conductivityenhancing chemicals and/or biological markers such as DNA may also beadded to the proppant composite for the purpose of easy identification,tracking, or other purposes. Additionally or alternately, a frac fluiddelay-crosslinking agent, such as betaine, gluconate, polyglycol, or acombination thereof, may be added. These chemicals may also be appliedas a mixture with the anti-attrition coating and be applied in Step 220and/or 230.

In Step 250, the modified proppant produced through the above processmay be stored and thereafter transported for use at a hydraulicfracturing site. It may also be possible to perform this process in situor anywhere in the supply chain, even including on demand at thehydraulic fracturing site. The material system, though, will showsubstantially improved dust control and attrition resistance performanceover untreated proppants and even proppants treated with alternativechemical systems.

FIG. 5 is a diagram of the use of a surface modified proppant inhydraulic fracturing. Hydraulic fracturing is commonly used in oil andgas production to maximize output from a wellbore 310. The processinvolves injecting a highly pressurized fluid 320, typically containingwater, chemicals, and proppants, into a wellbore 310, which causes theunderlying rock to crack. The proppants in the fluid then stay in thecracks in the rock and hold open the cracks, or fissures 340. Hydraulicfracturing is frequently used in combination with horizontal drilling330. By creating fissures 340 and filling them with materials (includingthe modified proppant of the present invention) to keep the fissuresopen, underlying hydrocarbons flow through the fissures into thewellbore for collection. Back in Step 220 of the process to make themodified proppant, the chemical coating 120 will not impede the flow ofthe modified proppant 100 as part of the highly pressurized fluid 320.The chemical coating 120 and resulting modified proppant 100 may also becompatible with the highly pressurized fluid 320, also called fracfluid.

While the composite is particularly suited for use as a proppant inhydraulic fracturing, it may be used in other applications in which lowdust/fines is desirable. For example, such a coating may be applied tocreate a low-dust, attrition-resistant composition for industrial sand(e.g., for use in glass, foundry, paint, construction applications),recreational sand (e.g., for use in playground, golf courseapplications), or for other minerals or powders.

The embodiments of this invention described herein are mainly toillustrate basic chemistries that could be employed to prepare aproppant composite with attrition resistance for achieving low proppantdust/fines and low respirable proppant dust/fines upon shipping andhandling, and especially upon pneumatic air unloading of such a proppantcomposite at a fracturing job site and/or upon use of other powderconveying, storage, or handling equipment. The chemistries employed inthis invention are chemicals that may be safe to humans and safe toaquatic species. Furthermore, these safe chemicals employed in thisinvention are also dominantly green and sustainable.

The following examples, used as illustration but not limitation,describe particular embodiments of the present invention.

Example 1. Conventional northern white quartz frac sand (20/40) wastreated with tall oil pitch at a level of 0.1 wt % of the frac sand. Thesurface temperature of the frac sand was 70 C and the temperature of thetall oil pitch was at 70 C or higher. The frac sand and tall oil pitchcoating were well mixed mechanically to achieve even coverage of talloil pitch on frac sand particulates. The finished product, a frac sandcomposite with tall oil pitch covering the surface of the frac sandparticulate, was then placed in a ball mill for a six-hour grinding atambient temperature to simulate real world conditions during a typicalshipment of the sand. The turbidity of the ground product was thenmeasured based on ISO 13503-2:2006E Section 9. The turbidity, 2 NTU, isshown in A) of FIG. 6. Un-coated frac sand was also put through thisgrinding process as described in this example, and its turbidity afterthe grinding was determined by the same ISO 13503-2:2006E Section 9testing protocol. The turbidity, 130 NTU, is shown in B) of FIG. 6 toserve as a control.

Example 2. As described in Example 1, a tall oil pitch and yellow greaseblend (50/50) was used to treat the frac sand at a level of 0.1 wt. % ofthe frac sand. The turbidity after the six-hour grinding, 32 NTU, isshown in C) of FIG. 6.

Example 3. Glycerin was used to treat the frac sand as described inExample 1 at 0.15 wt. % of the frac sand. Both frac sand and glycerinwere at ambient temperature. The turbidity after the six-hour grinding,32 NTU, is shown in D) of FIG. 6.

A six-hour abrasion study was also conducted, which showed there was asignificant difference in turbidity between the uncoated sand and coatedsand. Additional testing was done to ensure that the noticed differencein turbidity was not an artifact of the test protocol. This testing wasmeant to demonstrate that the observed reduction in fines was due toreduced attrition rather than embedding of fines in the coating.

In this study, the quantity of glycerine present in the water sampleused in the turbidity test was measured. The data showed that the coated0.15 wt. % of glycerin was entirely removed from the surface of thecoated sand. Therefore, the reduction in turbidity was due to reducedattrition rather than to capturing of dust/fines by the coating.

Further testing was done to demonstrate that the glycerin itself was notreducing the turbidity by, for example, agglomerating fines. In thisstudy, we also ran one test where we purposely added into the waterphase glycerin at a typical coating dosage and checked if the turbidityof the uncoated frac sand after six-hour abrasion was affected by thepresence of the glycerin. Our study showed that the addition of glycerininto the water phase at a typical coating dosage resulted in no changein the turbidity. Again, it pointed toward the fact that glycerincoating did improve the attrition resistance of a proppant.

Example 4. As described in Example 3, a glycerin/water blend (67/33) wasused to treat the frac sand at 0.15 wt. % of the frac sand. Theturbidity after the six-hour grinding, 32 NTU, is shown in E) of FIG. 6.

Example 5. As described in Example 3, a glycerin/water blend (50/50) wasused to treat the frac sand at 0.15 wt. % of the frac sand. Theturbidity after the six-hour grinding, 62 NTU, is shown in F) of FIG. 6.

Example 6. As described in Example 3, an industrial grade glycerin/waterblend (67/33) was used to treat the frac sand at 0.15 wt. % of the fracsand. The turbidity after the six-hour grinding, 28 NTU, is shown in G)of FIG. 6.

Example 7. As described in Example 3, a crude glycerin/water blend(67/33) was used to treat the frac sand at 0.15 wt. % of the frac sand.The turbidity after the six-hour grinding, 32 NTU, is shown in H) ofFIG. 6.

Example 8. As described in Example 3, a glycerin/water/propylene glycolblend (60/30/10) at 0.15 wt. % of the frac sand was used to treat thefrac sand. The turbidity after the six-hour grinding, 32 NTU, is shownin I) of FIG. 6.

Example 9. As described in Example 3, a glycerin/water/ethylene glycolblend (60/30/10) at 0.15 wt. % of the frac sand was used to treat thefrac sand. The turbidity after the six-hour grinding, 26 NTU, is shownin J) of FIG. 6.

Example 10. As described in Example 3, a glycerin/water/betaine blend(60/30/10) at 0.15 wt. % of the frac sand was used to treat the fracsand. The turbidity after the six-hour grinding, 26 NTU, is shown in K)of FIG. 6.

Example 11. As described in Example 3, a propylene glycol/water blend(67/33) was used to treat the frac sand at a dosage of 0.15 wt. % of thefrac sand. The turbidity after the six-hour grinding, 26 NTU, is shownin L) of FIG. 6.

Example 12. As described in Example 3, a glycerin/water/propyleneglycol/ethylene glycol/betaine blend (60/30/4/3/3) was used to treat thefrac sand at a dosage of 0.15 wt. % of the frac sand. The turbidityafter the six-hour abrasion, 24 NTU, is shown in M) of FIG. 6.

Example 13. Novolac resin coated frac sand (20/40) was coated with talloil pitch at 0.15 wt. % dosage. Both the substrate and the coatingtemperatures were at 70 C. After cooling down, the coated resin-coatedfrac sand and the un-coated resin-coated frac sand were subjected to12.5 hours of ball milling. The turbidities of both ball-milledproppants are shown in FIG. 7. The tall oil pitch coated resin-coatedfrac sand greatly reduced the turbidity of the resin-coated frac sandfrom 480 NTU to 76 NTU. Upon the same 12.5 hour ball milling, similarhigh degree of reduction in the turbidity of the tall oil pitch coatednorthern white sand (20/40; 0.15 wt. % dosage) from 870 NTU (uncoatedsand) to 50 NTU was also noticed, as shown in FIG. 7.

Example 14. Medium density ceramic (aluminum oxide) proppant was treatedwith tall oil pitch at 0.15 wt. % dosage. Both the substrate and thecoating temperatures were at 70 C. After cooling down, the coatedceramic proppant and the un-coated ceramic proppant were subjected to12.5 hours of ball milling. The turbidities of both ball-milledproppants are shown in FIG. 7. The tall oil pitch coating was able toreduce the turbidity of the ceramic proppant from 233 NTU to about 2NTU.

Example 15. The uncoated frac sand and the coated frac sand as describedin Example 1 and Example 4 were used for a pneumatic air unloading in ascaled down study. The scaled down study was conducted at about 12.5lbs/min sand pumping rate at 15 psi in a closed direct stream box.Samples in the middle of the uprising dust stream were collected on3-piece, 37 mm, pre-weighted PVC filter cassettes for a combination ofgravimetric and XRD analysis. A cyclone was used to collect particulatesin the respirable fraction. These collected samples were analyzed forrespirable quartz particulates, including quartz, and tridymite, andadditional respirable particulates (not just the silica fraction), basedon the modified NIOSH 0600/7500 and OSHA ID-142 methods. The respirablequartz dust levels are shown in FIG. 8. Un-coated frac sand generatedvery high levels of respirable quartz dust, while the coated frac sandsin this invention generated respirable quartz dust levels at least 94%lower than that of the uncoated frac sand. This closed direct stream boxtest was a very stringent test compared to a real job site situationwhere respirable quartz dust in the air is typically much more diffusedbefore it goes toward the workers.

Example 16. As described in Example 1, conventional northern whitequartz frac sand (#100, or 70/140) was treated with tall oil pitchcoating at 0.15 wt. % of the frac sand. The temperatures of both thefrac sand and tall oil pitch coating were at 100 C. The frac sand andtall oil pitch coating were well mixed mechanically to achieve evencoverage of tall oil pitch on frac sand particulates. The finishedproduct, a frac sand composite with tall oil pitch covering the surfaceof the frac sand particulate, was then placed in a ball mill for asix-hour grinding at ambient temperature. Uncoated quartz frac sand(#100, or 70/140) was also placed in a ball mill and ground for sixhours. The turbidities of both ground samples, 2 NTU and 170 NTU,respectively, are shown in N) and O) in FIG. 6.

Example 17. As described in Example 1, conventional northern whitequartz frac sand (20/40) was treated with tall oil pitch coating at 0.10wt. % of the frac sand. The temperatures of both the frac sand and talloil pitch coating were at 70 C. The frac sand and tall oil pitch coatingwere well mixed mechanically to achieve even coverage of tall oil pitchon frac sand particulates. The finished product, a frac sand compositewith tall oil pitch on frac sand particulates, was further coated with aglycerin based coating (67/33 glycerin/water blend) at 0.025 wt. % andmixed well mechanically at 70 C. The finished product was then placed ina ball mill for a six-hour grinding at ambient temperature. Theturbidity of the ground sample, 42 NTU, is shown in P) in FIG. 6.

Example 18. As described in Example 17, conventional northern whitequartz frac sand (20/40) was treated with tall oil pitch coating at 0.50wt. % of the frac sand. The temperatures of both the frac sand and talloil pitch coating were at 70 C. The frac sand and tall oil pitch coatingwere well mixed mechanically to achieve even coverage of tall oil pitchon frac sand particulates. The finished product, a frac sand compositewith tall oil pitch on frac sand particulates, was further coated with aglycerin based coating (67/33 glycerin/water blend) at 0.025 wt. % andmixed well mechanically at 70 C. The finished product was then placed ina ball mill for a six-hour grinding at ambient temperature. Theturbidity of the ground sample, 68 NTU, is shown in Q) in FIG. 6.

Example 19. Conventional northern white sand (20/40) was treated with acrude soybean oil/soybean oil wax blend (80/20 blend) at 0.50 wt. % ofthe frac sand. The frac sand and crude soybean oil/soybean wax blendcoating were well mixed mechanically at 70 C to achieve even coverage ofthe crude soybean oil/soybean wax coating on frac sand particulates. Theproduct was then further coated with a glycerin/water (67/33 blend)coating at 0.025 wt. % and mechanically well mixed at 70 C. The finishedproduct, a frac sand composite with coating covering the surface of thefrac sand particulates, was then placed in a ball mill for a six-hourgrinding at ambient temperature. The turbidity of the ground sample, 2NTU, is shown in R) in FIG. 6.

Example 20. Conventional northern white sand (70/140) was treated with atall oil pitch coating at 0.10 wt. % of the frac sand. The frac sand andthe coating were well mixed mechanically at 70 C to achieve evencoverage of the coating on frac sand particulates. The product was thenfurther coated with a glycerin/water (67/33 blend) coating at 0.025 wt.% and mechanically well mixed at 70 C. The finished product, a frac sandcomposite with coating covering the surface of the frac sandparticulates, was then placed in a ball mill for a six-hour grinding atambient temperature. The turbidity of the ground sample, 2 NTU, is shownin S) in FIG. 6.

Example 21. Conventional northern white sand (20/40) was treated with amethyl oleate coating at 0.10 wt. % of the frac sand. The frac sand andthe coating were well mixed mechanically at ambient temperature. Thefinished product was then placed in a ball mill for a six hour grindingat ambient temperature. The turbidity of the ground sample, 12 NTU, isshown in T) in FIG. 6.

Example 22. Conventional northern white sand (40/70) was heated up to100 C and then treated with glycerin/water (67/33; pre-mixed) at 0.125wt. % of the frac sand. Product was then mechanically mixed and placedin a ball mill for six-hour grinding at ambient temperature. Theturbidity of the ground sample is listed as A in FIG. 9.

Example 23. As described in Example 22, the 40/70 hot sand was treatedsimultaneously with separate additions of glycerin at 0.084 wt. % andwater at 0.041 wt. % of the frac sand. After mechanical mixing, thefinished product was then placed in a ball mill for six hour grinding atambient temperature. The turbidity of the ground sample, 09 NTU, islisted as B in FIG. 9.

Example 24. As described in Example 22, the 40/70 hot sand was treatedfirst with glycerin at 0.084 wt. % of the frac sand. After mechanicalmixing, the system was then further treated with water at 0.041 wt. % ofthe frac sand. After mechanical mixing, the finished product was thenplaced in a ball mill for six hour grinding at ambient temperature. Theturbidity of the ground sample, 66 NTU, is listed as C in FIG. 9.

Example 25. As described in Example 22, the 40/70 hot sand was treatedwith water at 0.041 wt. % of the frac sand. After mechanical mixing, thesystem was then further treated with glycerin at 0.084 wt. % of the fracsand. After mechanical mixing, the finished product was then placed in aball mill for six hour grinding at ambient temperature. The turbidity ofthe ground sample, 62 NTU, is listed as D in FIG. 9.

Example 26. As described in Example 22, the 40/70 hot sand without anychemical treatment was then placed in a ball mill for six hour grindingat ambient temperature. The turbidity of the ground sample, 178 NTU, islisted as E in FIG. 9.

Example 27. As described in Example 22, pre-blended hot (100 C) fracsand (45 wt. % of 20/40, 45 wt. % of 40/70 and 10 wt. % of 70/140) wastreated with glycerin/water (67/33) coating at 0.13 wt. %. Upon coolingdown, the treated frac sand was screened and the wt. % of each screensize was recorded. Two batch sizes were studied: 200 g pre-blended fracsand and 10 lb pre-blended frac sand. The pre-blended frac sand(uncoated) and the pre-blended coated frac sand (coated first beforeblending) were also screened to provide background data regardingparticle size distributions. The particle size distributions of thesefour systems is shown in FIG. 10.

Example 28. Conventional northern white sand (30/50) was treated withglycerin/water/KCl (66.4/32.7/0.9; pre-mixed) at 0.125 wt. % of the fracsand at ambient temperature. Product was then mechanically mixed andplaced in a ball mill for six-hour grinding at ambient temperature. Theturbidity of the ground sample, 68 NTU, is shown as A in FIG. 11. KClwas added to the coating as a marker by increasing the electricalconductivity of the wash-off liquid of the coated frac sand. Uncoatedconventional northern white sand (30/50) was placed in a ball mill forsix-hour grinding at ambient temperature. The turbidity of the groundsample was 167 NTU.

Example 29. As described in Example 28, the 30/50 sand was treated withglycerin/water/Rhodamine WT (67.00/32.99/0.01; pre-mixed) at 0.125 wt. %of the frac sand at ambient temperature. Product was then mechanicallymixed and placed in a ball mill for six-hour grinding at ambienttemperature. The turbidity of the ground sample, 57 NTU, is shown as Bin FIG. 11. Rhodamine was added to the coating as a UV marker forcomposite proppant.

Example 30. As described in Example 28, the 30/50 sand was treated withglycerin/water/Ecosphere 300 (Clariant) (66.77/32.32/2.91; pre-mixed) at0.125 wt. % of the frac sand at ambient temperature. Product was thenmechanically mixed and placed in a ball mill for six hour grinding atambient temperature. The turbidity of the ground sample, 52 NTU, isshown as C in FIG. 11. Ecosphere 300 was added to the coating as a colormarker for composite proppant.

Example 31. As described in Example 1, laboratory distilled water wasused to treat the frac sand at a level of 1.0 wt. % of the frac sand atambient temperature. The turbidity after the 12.5 hour grinding atambient temperature was over 800 NTU, over the scale. Water apparentlycan help to suppress dust but would not help to improve the attritionresistance of frac sand. Frac sand was also treated with tall oilpitch/yellow grease in a 50/50 blend at 0.10 wt. % at ambienttemperature and the turbidity after the 12.5 hour grinding was 13 NTU.

Example 32. A commercial northern white sand (20/40) coated withphenolic/formaldehyde cross-linked polymer was subjected to a 12.5 hourgrinding and the turbidity after the grinding was 480 NTU.

Example 33. A commercial northern white sand (40/70) coated withpropylene glycol/water (67/33) at 0.125 wt. % level of the frac sand wasused for a pneumatic air unloading in a scaled down study as describedin Example 15. Coated frac sand prepared based on Example 1 [northernwhite sand (20/40) coated with tall oil pitch at 0.10 wt. %] and coatedfrac sand based on Example 7 [northern white sand (20/40) coated withglycerin/water (67/33) at 0.15 wt. %] were also subjected to this sametype pneumatic air unloading test.

Example 34. A common northern white sand (70/140) was coated first witha tall oil pitch comprising coating [tall oil pitch/soybean oil (80/20)]at 0.125 wt. % followed by a glycerin-comprising second coating[glycerin/water (67/33)] at 0.005 wt. %. The coated frac sand was thensubjected to a pneumatic air unloading test as described in Example 33.

The percent total respirable dust reduction compared to the uncoatedfrac sand (70/140) was about 94%. The turbidity of the coated frac sandwas about 2 NTU or about 98% reduction compared to the uncoated one.

The percent total respirable dust (10 micron in size) reduction comparedto the uncoated frac sand for each coating is shown in FIG. 12. Over 90%reduction in total respirable dust generation was noticed on each coatedfrac sand. This is a direct measurement showing the improvement providedby this invention over conventional, uncoated frac sand. The reductionin total respirable dust translates to health benefits for the workerswho are responsible for handling the material and other persons near thesite, as well as environmental benefits for the plants, wildlife, andwater systems near the site.

There may be additional benefits to use of the material downhole.According to the literatures (SPE-171604-MS and Proppant Brief fromFairmountSantrol), dust/fines in the frac sand pack downhole contributedto the conductivity loss for the oil well. Among other things, theyfound that as little as 5% fines can reduce hydrocarbon flow rate up to60%. Halliburton has published similar findings, concluding that thecontrol of fines has proven to be the most valuable contributor toextending conductivity maintenance. The invasion of fines into aproppant pack can affect pack permeability, resulting inunderperformance and premature decline in well productivity.

FIG. 13 is a chart showing the total percent turbidity reduction factoracross many of the samples described in the examples. The TurbidityReduction Factor (TRF) can be expressed as a percentage according to thedifference between the turbidity of a ground uncoated sample and theturbidity of a ground coated sample, divided by the turbidity of theground uncoated sample, multiplied by 100 to yield percent, where allturbidity measurements are in common units such as NTU. TRF provides anindication of improvement in the surface modified proppant versus astandard, uncoated proppant, including the material's resilience to thecreation of dust/fines. It has been found that dust/fines are suppressedand other benefits are achievable where the TRF is at least 40%,preferably more than 60%, and most preferably more than 70%.

The Respirable Dust Reduction Factor (RDRF) is another indication ofimprovement in the surface modified proppant compared to standard,uncoated proppant. RDRF can be expressed as a percentage according tothe difference between the respirable dust of an uncoated sample and therespirable dust of a coated sample, divided by the respirable dust ofthe uncoated sample, multiplied by 100 to yield percent, where allrespirable dust measurements are in common units. It has been found thatdust/fines are suppressed and other benefits are achievable where theRDRF is greater than about 70%.

Whereas, the devices and methods have been described in relation to thedrawings and claims, it should be understood that other and furthermodifications, apart from those shown or suggested herein, may be madewithin the spirit and scope of this invention.

What is claimed is:
 1. A method of reducing proppant attrition anywherein a proppant supply chain, the method comprising using a surfacemodified proppant, where the surface modified proppant comprises: aproppant; and a chemical coating at least partially covering theproppant, where the chemical coating is not a thermoset polymer, not anionic polymer, not a thermoplastic elastomer, and not a hydrogel.
 2. Themethod of claim 1 where the proppant comprises raw substrate, includingsand, ceramic, or composite material, composites, minerals, groundshells, resin coated proppants, or combinations thereof.
 3. The methodof claim 1 where the chemical coating comprises glycerin-based coating,vegetable oil/wax-based coating, tall oil pitch based coating, alkylester based coating, or a combination thereof.
 4. The method of claim 1where the coating is non-petroleum-based, glycerin-based, propyleneglycol-based, or a combination thereof.
 5. The method of claim 1 wherethe coating is less than 2 wt. % of the surface modified proppant. 6.The method of claim 5 where the coating is less than 1 wt. % of thesurface modified proppant.
 7. The method of claim 6 where the coating is0.05 to 0.20 wt. % of the surface modified proppant.
 8. The method ofclaim 1 where the chemical coating is applied to the proppant throughmechanical mixing, spray, non-mechanical mixing, or a combinationthereof.
 9. The method of claim 1 where the coating comprises multiplecoatings and where the multiple coatings are applied simultaneously orsequentially onto the proppant.
 10. The method of claim 9 where themultiple coatings comprise a first coating and a second coating andwhere the first coating has a different chemical composition than thesecond coating.
 11. The method of claim 9 where the multiple coatingscomprise a first coating and a second coating, the first coating has achemical composition, the second coating has a chemical composition, andthe chemical composition of the first coating is the same as thechemical composition of the second coating.
 12. The method of claim 1where the coating is applied to the proppant prior to the proppant beingused.
 13. The method of claim 1 where the surface modified proppantfurther comprises a chemical marker.
 14. The method of claim 13 wherethe chemical marker is a colorant, a UV dye, a conductivity enhancingchemical, or a combination thereof.
 15. The method of claim 1 where thesurface modified proppant further comprises a frac fluiddelay-crosslinking agent.
 16. The method of claim 15 where the fracfluid delay-crosslinking agent is betaine, gluconate, polyglycol, or acombination thereof.
 17. The method of claim 1 where the coating doesnot require curing or drying.